You've had many threads started and received several answers... The last one started as a new thread looks to me as asking to justify a Safe t bar or maybe rather a safe-t-plus... people responded to your other threads singing praises thereof.

In another thread you already aligned it and that your tires were inflated correctly ... so if you're still unhappy ... I can only assume there's quite some doubt and floundering about what to do next.

My take:
Safe-T-Plus reduces the drivers tendency to feel yawning and therefor reduces the tendency to amplify it with back and forth steering wheel motions (See-Saw). For sure, that is a GOOD thing but it doesn't actually reduce the source of wag/yaw in the first place. FWIW... I had a Safe-T-Plus on previous E chassis and removed it because I didn't like fighting its return to center forces when driving roads it wasn't adjusted for. I found a heavier dampened stabilizer gave me the similar benefits without the return to center forces at play. With that said I would still consider a Safe-T-Plus stabilizer again due to our travel style having more primary type roads these days.

To reduce the source of tail wag the front to rear yawning must be tamed. Twist/Yaw happens from bushing play and spring flex. Unlike so many other solid axle vehicles with a panhard bar from the factory, our E chassis doesn't have one. There is nothing other than the spring packs themselves to prevent side to side flex/shift. The good news is that our front suspension does not have axle shift as a factor because each independent arm is much like a panhard bar by design. That leaves just bushings and rear axle for wag to be introduced.

Most Axis/Vegas units discussed here are fairly new so I'd think the bushings should be ok right? LoL ... anyway... the elephant in the room... a trac or panhard bar attaches to the frame and the axle to reduce side to side movement. It has little to no effect on up/down movement or body roll. Just controls that small amount of spring flex where the axle shifts left or right causing the vehicle to steer accordingly. A driver's corrections can start a see-saw or amplified yawning effect with force transfer oscillations back and forth as the driver corrals the vehicle down the road.

To reduce the source of tail wag the front to rear yawning must be tamed. Twist/Yaw happens from bushing play and spring flex. Unlike so many other solid axle vehicles with a panhard bar from the factory, our E chassis doesn't have one. There is nothing other than the spring packs themselves to prevent side to side flex/shift. ...cut....

I think it depends a lot on suspension type.

Track bars are common on solid rear axles when suspended by coil or air springs; where the suspension itself can't provide adequate lateral stiffness.

On the other hand factory vehicles with solid rear axles with leaf springs normally don't use a panhard bar (track bar) to prevent side-to-side suspension flex. Leaf springs are stiff laterally due to their geometry.

That's not to say that a track bar may not help some in reducing tail wag, but their disadvantages could outweigh advantages when used on vehicles with leaf springs. Ford may not use them on E-Series, but in fairness also don't use them on rear suspension of 2016 F-350 luxury pickups with leaf springs costing twice as much as an E-Series.

I know that travel arc can be a small factor in long travel suspension like that of a pickup. In the context here, on heavy laden short spring travel vehicle that only varies its carried weight duties a few percent, I'd be curious to hear about a disadvantage.

I know that travel arc can be a small factor in long travel suspension like that of a pickup. In the context here, on heavy laden short spring travel vehicle that only varies its carried weight duties a few percent, I'd be curious to hear about a disadvantage.

First, I would suggest that even if a motorhome doesn't change in overall weight very much (which I agree), it doesn't mean the suspension won't go through almost as much travel. Spring stiffness is set at factory in proportion to maximum expected axle load so that the ride isn't too stiff. Pickup trucks when empty do ride higher and often too stiff at back, but full suspension travel isn't that different for many other reasons. On large bumps a motorhome's rear suspension could momentarily unload a bunch, allowing the track bar to pull axle to one side or other (depending on initial angle). Another unintended consequence is body roll which changes travel arc and thus apply lateral force on axle. However, if that were the main issue you could go with a better set up than a track bar that eliminates lateral movement; but we digress.

I think the real reason track bars are not normally used with solid axle leaf spring suspensions is that if you do the math, a 3-inch-wide steel leaf just under 5-feet long can't flex laterally enough to worry about. By comparison, for example, tires would flex much more.

My personal concern is that a track bar could load the leaf springs laterally due to the travel arc and the stiff leaf springs not being able to move as much laterally.

Let's not forget that there is also an inherent travel arc of the axle and leaf springs versus frame, so that when the vehicle rolls the springs can split the difference between the two. When a track bar is added and the vehicle body rolls, one spring will likely get loaded less at the expense of the other getting loaded more. And that's not a good thing.

If track bars were needed and there were not some disadvantages when combined with leaf springs, I think OEMs would include them from factory because their weight and cost are relatively low.